The iron pipe used as a water supply pipe is apt to corrode and form sludge on its inner surface and its service life is short.
Owing to such disadvantages of the iron pipe, the use of a stainless steel pipe light in weight and superior in corrosion resistance has recently been taken note of. Stainless steel pipes are thin-walled in comparison with their diameters and so cannot be threaded for coupling at their end portions. For this reason it is impossible to adopt the conventional coupling structure and it is necessary to provide a special coupling.
Essential functions required of a coupling for a thin-walled pipe are to prevent movement in the axial direction of the pipe, that is, to prevent disengagement of the pipe, and to ensure watertightness against positive and negative pressures in the pipe.
A thin-walled pipe coupling has heretofore been proposed and known in U.S. Pat. No. 4,438,954, in which the outer periphery of the pipe is clamped with a C-shaped clamp ring to prevent disengagement of the pipe. In this structure, however, the disengagement preventing effect is extremely insufficient; besides, not only the clamp edge can damage the outer periphery of the pipe, making it impossible to re-use the pipe, but also there is fear of deformation of the pipe because the clamping force is not uniformly applied to the pipe outer periphery.
Further, there has also been proposed a thinwalled pipe coupling of a structure in which a pipe end is reduced in diameter inwards and a packing is brought into abutment with the outer tapered surface to provide a sealing surface. However, this coupling has an essential drawback that a slight change in pipe length immediately leads to the loss of sealability.